Devices of this type are often required on automobiles to monitor the rotational behavior of the individual vehicle wheels. Wheel speed is of critical importance as an input variable for control systems, such as anti-lock systems, traction slip control systems and driving stability control systems.
Sensor devices to determine the rotational speed of wheels are known in a great number of applications. Normally, the measuring data emitter in such sensor devices is an incremental encoder which is coupled mechanically to the component or wheel the rotation of which is to be measured. Further, there is a transducer or sensor which scans the encoder. Ferromagnetic toothed wheels, toothed rings, ferromagnetic hole discs, etc., are used as encoders. When sensors are used in wheel bearings, it is also customary to employ magnetized structures as a measuring data emitter, for example, annular or circular arrangements of successive north/south poles, embedded in a mechanic carrier member.
It is also common to use so-called "passive" sensors according to the reluctance principle. These sensors use a copper coil with a permanent magnet as a transducer. The transducer is coupled magnetically to the toothed disc serving as measuring data emitter, or to any other encoder. The encoder modulates the magnetic coupling reluctance synchronously with movement, and an alternating voltage representative of the movement is induced in the copper coil. The frequency of the alternating voltage permits being evaluated as a measured quantity to determine the rotational wheel speed. The magnitude of the induced signal voltage is a function of the rotational speed and the air slot between the measuring data emitter and the transducer or between the tooth system and the sensor.
"Active" sensors, which also cover the subject matter of the present invention, are well known in the art. In principle, they are combined of a magneto-statically sensitive element and a permanent magnet which is magnetically coupled to the encoder. The encoder modulates, synchronously to movement, the magnetic coupling reluctance or the field direction, and the sensor element responds to the variation of the flux density or the movement of a field vector. Examples in the art of such magneto-statically responsive elements are Hall probes and magneto-resistive structures on the basis of permalloys. The magnitude of the signal voltage on the sensor element is responsive to the air slot, but independent of the rotational speed or the frequency.
The publication entitled "Magneto-resistive rotational speed sensor--reliable and inexpensive" (Graeger, Petersen, Elektronik 24/1992, pages 48 to 52) describes an active rotational speed sensor of such a type which interacts with a toothed disc made of ferromagnetic material that is the measuring data emitter. The actual sensor element includes a permanent magnet on the side remote from the toothed disc. The magnetic field of this magnet serves to bias the magneto-resistive sensor element and to produce the magnetic coupling reluctance with the co-rotating toothed wheel. Therefore, a permanent magnet with a relatively large volume is required to permit an air slot that is sufficient in practical operations.
An object of the present invention is to provide a device of the previously mentioned type including an active sensor element which, compared to known devices of this type, necessitates a minimum possible structural volume to permit accommodation of the device in a wheel bearing of an automotive vehicle, for example, and which also permits a largest possible air slot or distance between the measuring data emitter and the transducer.
A special feature of the device of the present invention is that the measuring data emitter has successive permanent magnet areas of alternating polarity in the direction of rotation, and that the sensor element is arranged and the bias magnet is magnetized such that the bias magnet produces a field component which extends vertical to the direction of movement of the measuring data emitter in a preferred direction of the sensor element, and that during a rotary movement, in a preferred direction of the sensor element which is orthogonal to the magnetic field component produced by the bias magnet, the magnet areas cause a varying course of field strength between the adjacent permanent magnet areas which passes through the sensor element in the direction of rotation and represents the rotary movement.
A magneto-resistive sensor element is used in the present invention because it is known to permit, under comparable conditions, larger air slots between the encoder and the sensor compared to Hall elements. The sensor element KMI 10/1 cited in the above-mentioned publication (Elektronik 24/1992) is an example of a like active rotational speed sensor element. The above-mentioned sensor element is intended for use in combination with an encoder of any ferromagnetic material. As is shown in the attached FIG. 1, the sensor comprises a magneto-resistive resistance bridge 1, an electronic evaluating circuit 2 and a permanent magnet 3 which is magnetized in the direction of the XZ plane. In operation, the sensor is coupled magnetically over a small air slot to a toothed wheel made of ferromagnetic material. The teeth are oriented in the X direction (see FIG. 1) and move on rotation of the toothed wheel in the Y direction past the side of the bridge, comprising the magneto-resistive resistors, that is opposite to the permanent magnet 3. The result is an alternating deformation of the magnetic field of the XZ plane additionally in the Y direction. The magneto-resistive bridge is so designed and arranged that it reacts to the field strength component in the Y direction by mistuning. The electronic evaluating circuit 2 mainly comprises a bridge signal amplifier with a subsequent trigger circuit which produces a binary output signal with two constant amplitude values in the area of the nominal air slot irrespective of the size of the air slot. The change in flanks of the amplitude values represents the division of the toothed wheel of the measuring data emitter. The evaluating circuit is designed so as to issue the signal in the form of a current through terminals 4. Both the sensor element and the signal-processing circuit are integrated circuits that are encapsulated in a plastic housing each. The two housings are mechanically interconnected by a system carrier member 10. Also, the system carrier member provides the electrically conductive connections.
The device of the present invention is based on the teaching that the desired reduced structural, volume and, additionally, a larger admissible air slot can be achieved by subdividing the magnetic structure into a bias magnet, which produces a magnetic field in the X direction, that is, in one of the preferred magnetic directions of the magneto-resistive sensor element, which extends orthogonally to the direction of movement of the encoder (Y direction), and the permanent magnet areas of a measuring data emitter.
In a preferred embodiment of the device of the present invention, the measuring data emitter has the shape of a disc performing the rotary movement. More preferably, the disc-shaped emitter is cylindrical in shape having an inner and outer periphery. The permanent magnet areas are distributed evenly over the periphery of the disc. Preferably, the permanent magnet areas are embedded on the outer edge periphery of the disc. Alternatively, the permanent magnet areas are embedded on the inner periphery of the disc. Appropriately, the permanent magnet areas are embedded in a mechanic carrier material or they are produced by magnetization of areas. The transducer is aligned in parallel to the disc or paraxially depending on the arrangement of the areas.
Further favorable embodiments of the present invention are set forth herein. For example, the transducer may be configured as a one-housing or a two-housing basic element, and the bias magnet may be arranged on the housing, especially on the side remote from the encoder, or it may be embedded into the housing.